The present invention relates to a method for pelletizing synthetic resins, and more particularly to a method for pelletizing synthetic resins having a relatively high melting point, e.g. 200.degree.C. or more, by an under-water cut process using extruders.
It has been long known that, if the length (1) of the die is selected to be more than 8 times the diameter d of the die nozzle, the synthetic resins are extruded in laminar flow without being subjected to melt fracture, Barns effect or nozzle end effect, etc.
However, in reality, the length of the die varies due to its provision with a die jacket, as well as the construction to resist the resin pressure. Further, it varies also with the dimension of the extruders.
Accordingly, the length of the die is generally selected to be 10 to 15 times the diameter of the die nozzle and, at the same time, the resin pressure is considered when determining the land width of the die nozzle.
Recently, a remarkable development has been made in the field of the synthetic resin industry, and various kinds of very excellent synthetic resins have been produced. Among them are resins which have relatively high melting points compared with the conventional ones. These high melting point resins include, for example, nylon, polyester resin, etc.
Further, in the case of supplying synthetic resins from the maker to the processor, it has now become common practice to deliver them in pelletized form, because handling is convenient and the packing and transporting costs become more economical as the materials' bulk specific gravity becomes smaller.
In this point, there are many processes for pelletizing the synthetic resins, typical ones being listed as follows:
A. sheet cut process; PA1 B. strand cut process; PA1 C. hot cut process; and PA1 D. under-water cut process.
In the case of pelletizing synthetic resins having a high melting point such as stated above, hitherto either the sheet cut or the strand cut process has been generally adopted. However, neither can achieve satisfactory pelletization of such resins due to the fact that their viscosity is relatively low in their melted state. Thus, the control of the cooling velocity or taking up velocity, etc., of sheet-form or strand-form resins after the extrusion through nozzles of a die is so difficult that the thickness of sheets or the diameter of strands is subject to variation. Pellets produced from such sheets or strands become uneven and breaks often occur during the take-up operation. Further, these cut processes, also known as a cold cut process, require a number of manual manipulations in conducting the resin out of the die through the cooling water tank to the cutting device along with noise therein, among other defects.
Further, the resin industry is recently tending toward labor saving through development of equipment with a large processing capacity. For this purpose, it has been well-known that the under-water cut process is the most suitable one. However, it has been regarded as impossible to pelletize the synthetic resins having a high melting point by such an under-water cut process, because it has been developed solely for pelletizing synthetic resins having a relatively low melting point, so that nobody has ever contemplated to use such a process for this purpose. The reason for the nonuse of this under-water cut process is that since the die surface is always under water, warm or cold, the temperature of the surface of the die is very low compared with the melting point of the resins having a high temperature, so that the resins set and clog the nozzles of the die. Finally, in order to obtain pellets having a diameter of 3 to 3.5 mm, as generally marketed, the die nozzles have a diameter of 2 to 2.4 mm.